When a highly reactive tuned load is driven by a signal at a variable frequency, especially a frequency shift keyed (FSK) signal, the output current will be limited if the resonant frequency of the tuned load does not match the frequency of the drive signal. Significant energy can also be wasted if the phase of the energy in the tuned load for a particular bit is not synchronous with the phase of the input signal for a subsequent bit, i.e. at the bit transitions.
There are many applications in which it may be desirable to drive a narrow-bandwidth tuned load with an amplified FSK signal, such as in the generation and modulation of AC magnetic fields by transmitter. Such fields are used in magnetic inductive systems, which are employed for purposes of navigation, communication, signaling, direction finding and other applications.
The quasi-static AC magnetic fields used in magneto-inductive systems are typically generated by driving a low-frequency AC current through a tuned antenna formed from electromagnetic loop coils or a solenoid. Power losses are minimized by ensuring that the antenna load is a high-Q load. This has the effect of making the antenna load a highly-tuned narrow-bandwidth load, thereby making it difficult to drive effectively unless the resonant frequency of the load is matched precisely to the frequency of the drive signal, and the phase of the load current is not discontinuous with respect to the phase of the drive signal. When the drive signal has a variable frequency, such as in the case of an FSK drive signal, difficulties arise in ensuring that the resonant frequency of the load precisely tracks the frequency changes of the drive signal at bit transitions. Additionally, changes in tuning components versus temperature, for example, can cause detuning of the load as the ambient temperature changes.